1. Field of the Invention
The present invention relates to the directional transmission of sound, and more particularly to the directional transmission of sound using an array of transducers.
2. Description of the Related Technology
The ability to focus sound, that is to have the strength of sound be stronger in a certain region with respect to all other regions, has many uses. Mechanisms exist today for producing a directional or localized sound.
Transducers, in general, are devices which convert one kind of energy into another kind of energy. An example is a microphone, which converts an acoustic wave (changes in air pressure) into an electrical signal. The converse, which converts an electrical signal into an acoustic wave, is called a loudspeaker. Loudspeakers can be characterized by the band of acoustic frequencies they can faithfully reproduce, i.e. reproduce at a sufficient volume with minimal distortion. The range of frequencies audible to the average person has been estimated between 20 Hz and 20 kHz. Broadband loudspeakers attempt to faithfully reproduce all of these frequencies, but, practical broadband loudspeakers generally faithfully reproduce frequencies between 100 Hz and 10 kHz.
In addition to producing different frequencies, some loudspeakers are designed to transmit sound directionally. One type of directional loudspeaker is one which uses a dipole source, in which sound in transmitted in two directions, forwards and backwards. This is in contrast to an omni-directional source, which produces sound in all directions. Structure may also be used to affect the direction of sound transmission. For example, loudspeakers with spherical, parabolic, or elliptical reflectors are known to form directional sounds beams. The reflector, which may be placed behind the loudspeaker, causes sound to be transmitted forward to propagate unimpeded, and causes sound propagating backward to be reflected forward by the reflector. However, reflector-based directional speakers are limited in their low frequency response by the diameter of the reflector. Shadowing of the reflector by the source loudspeaker also interferes with the directionality, that is, as sound is reflected off the reflector forward, it may be impeded by the source. Horns with an exponential or conical cross-section can also be used to form directional sound beams. However, as the directionality of horn loudspeakers is determined by the dimensions of the mouth of the horn and internal contour, they must be very large to achieve high directionality. Another method of creating a directional beam of sound is to use a parametric array, which takes advantage of the non-linearity of air at high sound pressures to demodulate and amplitude modulated ultrasonic carrier. This requires that the speaker generate a relatively high intensity, typically greater than 130 dBspl in the 40-70 kHz range, which may be unfavorable or unsafe for producing audio for a human listener.